A power supply apparatus that uses a buck converter (a step-down converter) is known as an apparatus for lighting a discharge lamp. In a discharge lamp, there is a transition to steady state arc discharge via a glow discharge stage after breakdown. This means that the power supply apparatus needs to supply a high voltage to achieve glow discharge at the start of illumination. In addition, the power supply apparatus needs to supply a sufficient power to maintain the arc discharge at the transition from glow discharge to arc discharge.
In Japanese Laid-Open Patent Publication No. 2000-123989, a discharge lamp lighting device is disclosed where the capacitance of a smoothing capacitor is reduced at the start of illumination so that sufficient energy is supplied, and then in the steady state following the transition to arc discharge, the capacitance of the capacitor is increased to reduce high-frequency ripple components.
Among power supply apparatuses that include a buck converter, apparatuses that do not have a boost (i.e., step-up) function have a simple construction and can be made compact, but are incapable of outputting a higher voltage than the input voltage. In the case of an HID lamp used as a light source of a projector or the like, it is necessary to supply a voltage of around 200V during the glow discharge stage at the start of illumination. It is also necessary to illuminate the HID lamp stably every time, even if the input voltage is low.
PWM (pulse-width modulation)-type buck converters are often used in power supply apparatuses. By using such type of buck converter, in addition to the merit of regulating the power by changing the duty of PWM pulses with a constant frequency and having a proven record in actual use, it is possible to operate with a high frequency to prevent resonance for an HID lamp. However, in a continuous mode where a current flows continuously through the choke coil (i.e., inductor), as shown by Equation (1) below, it is only possible to output an output voltage Vo determined by the duty ratio of the (Ton/(Ton+Toff)) of the input voltage Vi (Ton is a time where a switch is on by the PWM pulses and Toff is a time where the switch is off by the PWM pulses). In a buck converter where condition such as the inductance L of the choke coil, is selected as to produce a discontinuous mode (intermittent mode) for a current Io of the magnitude that flows during glow discharge, current flows discontinuously (i.e., intermittently) through the choke coil and a higher output voltage Vo than the duty ratio of the input voltage Vi can be outputted as shown by Equation (2) below.Vo=Ton/(Ton+Toff)×Vi  (1)Vo=(Vi·Ton)2/(Vi·Ton2+2·Io·L(Ton+Toff))  (2)
Equations (1) and (2) above respectively show the output voltage Vo in continuous mode (where Io>(Vo/(2·L)·Toff)) and the output voltage Vo in the discontinuous mode (where Io<(Vo/(2·L)·Toff)) of a typical buck converter.
During the transition from glow discharge to arc discharge, the current Io increases due to the addition of a thermal electron flow, and as a result, the discharge voltage (i.e., the output voltage) Vo falls. During the transition, the current supplying performance of the power supply apparatus in a high-voltage range has a large effect on the ease with which the lamp can be lit. However, it is difficult to achieve both a high output voltage Vo and a high output current Io in discontinuous mode. When the output current Io is increased, the output voltage Vo falls, and it also becomes easy for the buck converter to change to operating in continuous mode.